Exposure apparatuses for semiconductor processing are commonly used to transfer images from a reticle onto a semiconductor wafer during semiconductor processing. A typical exposure apparatus includes an illumination source, a reticle stage assembly that positions a reticle, an optical assembly having an optical axis, a wafer stage assembly that positions a semiconductor wafer, a measurement system, and a control system. One type of stage assembly includes a stage base, a stage that retains the wafer or reticle, and a stage mover assembly that moves the stage and the wafer or the reticle. The measurement system constantly monitors the position of each stage and the control system controls each stage mover assembly to constantly adjust the position of the reticle and the wafer. The features of the images transferred onto the wafer from the reticle are extremely small. Accordingly, the precise positioning of the wafer and the reticle is critical to the manufacturing of high quality wafers.
In certain designs, the measurement system includes an autofocus assembly that monitors the position of the wafer along the optical axis. Subsequently, with information regarding the position along the optical axis, the stage mover assembly can be controlled to properly position the wafer along the optical axis.
One type of autofocus assembly includes a slit light source that illuminates a set of slits and an imaging system that projects the set of slits onto the wafer. The light reflected from the wafer is then directed into a receiving imaging system that projects an image of the slits onto a second set of slits. The light that passes through the second set of slits is subsequently measured by a slit detector assembly. With information from the slit detector assembly, the position of the wafer along the optical axis can be determined.
The autofocus assembly discussed above utilizes the plurality of slits of light reflected off of the wafer to determine the position of the wafer along the optical axis. Unfortunately, the reflectivity of the wafer varies along the surface of the wafer. For example, circuits and other features on or below the wafer surface can influence the reflectivity of the wafer at that area. The variable reflectance of the wafer can adversely influence the light reflected off of the wafer. As a result thereof, the position of the wafer perpendicular to the optical axis influences the measurements performed by the autofocus assembly and the accuracy of the measurements obtained by the autofocus assembly.